1. Field of the Invention
The present invention relates generally to methods for noninvasively determining the pulmonary capillary blood flow (“PCBF”) or cardiac output (“CO”) of a patient. Particularly, the present invention relates to so-called differential Fick techniques for determining PCBF or CO including partial rebreathing techniques.
2. Background of the Related Art
Pulmonary capillary blood flow and cardiac output are examples of various hemodynamic parameters that may be monitored in critically ill patients. Cardiac output is the sum of blood flow through the lungs that participates in gas exchange, which is typically referred to as pulmonary capillary blood flow, and the blood flow that does not participate in gas exchange, which is typically referred to as intrapulmonary shunt flow or venous admixture.
Conventionally, pulmonary capillary blood flow and cardiac output have been measured by direct, invasive techniques, such as by indicator dilution. Indicator dilution includes introducing a predetermined amount of an indicator into the bloodstream through the heart of a patient and analyzing blood downstream from the point of introduction to obtain a time vs. dilution curve. Thermodilution, in which room temperature or colder saline solution, which is also referred to as “cold” saline, is used as the indicator, is a widely employed type of indicator dilution. The cold saline is typically introduced into the right heart bloodstream of a patient through a Swan-Ganz catheter, which includes a thermistor at an end thereof The thermistor is employed to measure the temperature of the blood after it has passed through the right heart, or downstream from the point at which the cold saline is introduced. A thermodilution curve is then generated from the data, from which the cardiac output of the patient may be derived. Thermodilution and other indicator dilution techniques are, however, somewhat undesirable due to the potential for harm to the patient that is associated with inserting and maintaining such catheters in place.
Less invasive indicator dilution methods that do not require that a catheter pass through the valves of the right side of the heart have also been developed. These less invasive methods include the so-called “transpulmonary indicator methods,” which include the placement of probes in the esophagus or trachea (e.g., in Doppler/Transesophageal echocardiography). While the use of esophageal or endotracheal probes may seem less invasive than the introduction of a catheter into the heart of a patient, the potential for harming a patient exists nonetheless.
Thus, safer, noninvasive techniques for determining pulmonary capillary blood flow and cardiac output have been developed. These noninvasive techniques are typically based on some form of the basic physiological principle known as the Fick principle: the rate of uptake of a substance by the blood or release of a substance from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung.
One variation of the Fick principle is the so-called carbon dioxide Fick equation:Qpcbf=VCO2/(CvCO2−CaCO2),  (1)where Qpcbf is pulmonary capillary blood flow, VCO2 is carbon dioxide elimination, CvCO2 is carbon dioxide content of the venous blood of the patient, and CaCO2 is the carbon dioxide content of the arterial blood of the patient.
Typically, a differential form of the carbon dioxide Fick equation is used to noninvasively determine the pulmonary capillary blood flow or cardiac output of a patient. Each of the differential Fick techniques for determining the pulmonary capillary blood flow or cardiac output of a patient are based on the fundamental premise that pulmonary capillary blood flow and cardiac output can be estimated based on the changes of other, measurable parameters when a change in the effective ventilation (i.e., the total ventilation less the wasted ventilation due to deadspace associated with the apparatus, the patient, or a combination thereof) occurs. When a differential form of the Fick equation is used, the pulmonary capillary blood flow or cardiac output of a patient may be determined on the basis of differences in VCO2, CaCO2, and CvCO2 between “normal” respiration and while a change in the effective ventilation of the patient is being induced. The following is an example of a differential Fick equation:
                                          Q                          p              ⁢                                                          ⁢              c              ⁢                                                          ⁢              b              ⁢                                                          ⁢              f              ⁢                                                          ⁢              B              ⁢                                                          ⁢              D                                =                                                    V                                  C                  ⁢                                                                          ⁢                                      O                                          2                      ⁢                      B                                                                                  -                              V                                  C                  ⁢                                                                          ⁢                                      O                                          2                      ⁢                      D                                                                                                                          (                                                                            C                      v                                        ⁢                    C                    ⁢                                                                                  ⁢                                          O                                              2                        ⁢                        B                                                                              -                                                            C                      v                                        ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                                          O                                              2                        ⁢                        D                                                                                            )                            -                              (                                                                            C                      a                                        ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                                          O                                              2                        ⁢                        B                                                                              -                                                            C                      a                                        ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                                          O                                              2                        ⁢                        D                                                                                            )                                                    ,                            (        2        )            where VCO2B and VCO2D are, respectively, the carbon dioxide eliminations of the patient during “normal” breathing and while a change in the effective ventilation of the patient is being induced, CvCO2B and CvCO2D are the contents of carbon dioxide in the venous blood of the patient during the same periods, and CaCO2B and CaCO2D are the contents of carbon dioxide in the arterial blood of the patient during “normal” breathing and when the effective ventilation of the patient is changed, respectively.
Typically, differential Fick techniques rely upon baseline measurements (i.e., taken during “normal” respiration) of VCO2 and PetCO2. Once baseline data has been gathered, a change in the effective ventilation of the patient is induced. Once the VCO2 and PetCO2 values become stable with the change in effective ventilation, these parameters are again measured. The difference between the baseline values and those taken during the change in the effective ventilation of the patient are used to calculate the pulmonary capillary blood flow or cardiac output of the patient. When continually monitoring and updating the pulmonary capillary blood flow or cardiac output of a patient, a recovery period typically precedes reestablishment of the baseline values. The recovery period has been provided to facilitate the reestablishment of baseline levels prior to the start of rebreathing. Most investigators do not collect any data for analysis during the recovery period.
The carbon dioxide Fick equation (1) and the differential Fick carbon dioxide equation (2) each require a determination of the VCO2 of a patient. Carbon dioxide elimination is the net volume of carbon dioxide produced by the patient, or excreted from the body of a patient, during respiration. Therefore, carbon dioxide elimination is useful as an indicator of the metabolic rate of the patient. The VCO2 of a patient may be noninvasively measured as the difference, per breath, between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration. Carbon dioxide elimination over a breath is typically calculated as follows:VCO2=∫breathV×fCO2dt,  (3)where V is the measured respiratory flow and fCO2 is the substantially simultaneously detected carbon dioxide signal, or fraction of the respiratory gases that comprises carbon dioxide, or “carbon dioxide fraction.”
During rebreathing, the exhaled volume of carbon dioxide may change only slightly, while the inhaled volume of carbon dioxide, which is normally negligible, may increase substantially. As a consequence, the difference between the amounts of carbon dioxide that are exhaled and inhaled during rebreathing is reduced substantially, as is the carbon dioxide elimination of a patient.
A determination of the CaCO2 of a patient is typically based upon the measured PetCO2 of the patient. The PetCO2, after correcting for any deadspace in the patient's airway or in a ventilation circuit, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO2) of the patient's lungs or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO2). Using a standard carbon dioxide dissociation curve, either the PetCO2 measurement or the PaCO2 calculation may be used to determine CaCO2.
When a change in the effective ventilation of a patient occurs, such as when a patient inhales increased concentrations of carbon dioxide, CaCO2 changes relatively quickly compared to the rate of change in CvCO2, which has a higher carbon dioxide content CaCO2. The content of carbon dioxide in the venous blood also changes relatively slowly because the body stores a large volume of carbon dioxide in other tissues. The carbon dioxide stores of an “average” human male may be as high as about 15 to 40 liters. Thus, the duration or magnitude of a change in the effective ventilation of a patient must be significant (e.g., the patient must inhale a significant amount of carbon dioxide) to effect a measurable change (e.g., increase) in the content of carbon dioxide in the patient's venous blood. Likewise, the VCO2 of a patient changes at a faster rate than CvCO2. In fact, CvCO2 changes so slowly relative to the rates at which CaCO2 and the VCO2 change that, when a change in effective ventilation occurs over a relatively brief period of time (e.g., a few minutes or less), the CvCO2 can be assumed to remain substantially the same (i.e., little or no change) over the time it takes to complete a conventional rebreathing maneuver. As the effects of rebreathing are delayed due to transport time and damping, conventional rebreathing processes typically employ a recovery period.
Carbon dioxide elimination and the PetCO2 are typically measured during both of the phases of a differential Fick technique.
In one example of a known differential Fick technique for inducing a change in the effective ventilation of a patient, carbon dioxide may be added to the gases that are inhaled by the patient, either directly (e.g., by the addition of carbon dioxide from a cylinder or other external source) or by causing a patient to rebreathe previously exhaled gases. An exemplary differential Fick technique that has been employed, which is disclosed in Gedeon, A. et al. in 18 Med. & Biol. Eng. & Comput., 411–418 (1980) (hereinafter “Gedeon”), employs a period of increased ventilation followed immediately by a period of decreased ventilation. When the technique disclosed in Gedeon or another so-called “rebreathing” process is used, the VCO2 of the patient decreases to a level that is less than that which is measured during normal breathing. Rebreathing during which the VCO2 decreases to near zero is typically referred to as “total rebreathing.” Rebreathing that causes some decrease, but not a total reduction of VCO2, is typically referred to as “partial rebreathing.” These rebreathing processes may be used either to noninvasively estimate the CvCO2, as in “total rebreathing,” or to obviate the need to know CvCO2, as in “partial rebreathing.”
Rebreathing is typically conducted with a rebreathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. For example, the rebreathed air, which may be inhaled from a deadspace during rebreathing, includes air that has been exhaled by the patient (i.e., carbon dioxide-rich air).
During total rebreathing, substantially all of the gas inhaled by the patient was expired during the previous breath. Thus, during total rebreathing, PetCO2 is typically assumed to be equal or closely related to the partial pressure of carbon dioxide in the arterial (PaCO2), venous (PvCO2), and alveolar (PACO2) blood of the patient. Total rebreathing processes are based on the assumption that neither the pulmonary capillary blood flow or cardiac output, nor the CvCO2 of the patient, changes substantially during the rebreathing process. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve, where the change in the carbon dioxide content of the blood (CvCO2—CaCO2) is equal to the slope(s) of the carbon dioxide dissociation curve multiplied by the measured change in PetCO2, as caused by a change in effective ventilation, such as rebreathing.
In partial rebreathing, the patient inhales a mixture of “fresh” gases and gases that were exhaled during the previous breath. Thus, the patient does not inhale a volume of carbon dioxide as large as the volume of carbon dioxide that would be inhaled during a total rebreathing process.
As an example of a known partial rebreathing process, the NICO™ system offered by Novametrix Medical Systems Inc. of Wallingford, Conn., employs a 60 second baseline period, a 50 second rebreathing period, and a 70 second recovery period. The complete rebreathing cycle lasts for about three minutes. Another exemplary partial rebreathing process is disclosed in Capek, J M, and Roy, R J, Noninvasive measurement of cardiac output using partial CO2 rebreathing, IEEE Trans. Biomed. Eng. 1988; 35:653–661. That rebreathing process has a total cycle time of about 3½ minutes, with the actual rebreathing phase lasting for about 30 seconds. Gama de Abreu, M, et al., Partial carbon dioxide rebreathing: A reliable technique for noninvasive measurement of nonshunted pulmonary capillary blood flow, Crit. Care Med. 1997; 25: 675–683, discloses a rebreathing process with a 35 second rebreathing phase and a total cycle time, including baseline and recovery phases, of about 3 minutes.
Conventional partial rebreathing processes typically employ a differential form of the carbon dioxide Fick equation, such as equation (2), to determine the pulmonary capillary blood flow or cardiac output of the patient without requiring knowledge of the carbon dioxide content of the venous blood of the patient since the carbon dioxide content of the venous blood of the patient is assumed to remain substantially the same (i.e., constant) in the periods during which measurements are obtained.
Again, with a carbon dioxide dissociation curve, the measured partial pressure of end tidal carbon dioxide can be used to determine the change in content of carbon dioxide in the blood before and during the rebreathing process. Accordingly, the following equation can be used to determine pulmonary capillary blood flow or cardiac output when partial rebreathing is conducted:Q=ΔVCO2/sΔPetCO2,  (4)where s is the slope of the carbon dioxide dissociation curve.
While partial rebreathing is the most commonly used method for causing a change in the effective ventilation of a patient, alternative differential Fick techniques for measuring pulmonary capillary blood flow or cardiac output have also been employed. Such differential Fick methods typically include a brief change of PetCO2 and VCO2 in response to a change in effective ventilation. This brief change can be accomplished by adjusting the respiratory rate, inspiratory and/or expiratory times, tidal volume, inspiratory pause, or positive-end expiratory pressure (PEEP) of the patient's respiration.
While many existing differential Fick techniques provide reliable, noninvasively obtained measurements of pulmonary capillary blood flow or cardiac output, the lengths of time over which these techniques are effected are somewhat undesirable, especially in the common critical and intensive care situations in which it is desirable to more frequently update measurements of the pulmonary capillary blood flow or cardiac output of a patient.
Accordingly, there is a need for a method of noninvasively calculating pulmonary capillary blood flow and cardiac output with increased frequency.